The present invention relates to liquid supply pumps, and more particularly to linearly reciprocable liquid supply pumps for pumping liquids having thermosetting characteristics, wherein accumulations of such liquids which dry out and become set can impair the operability of the pump.
Linearly reciprocable liquid pumps have long been used in industry for a wide range of liquid delivery applications. For example, the assignee of the present invention manufactures a complete line of reciprocable pumps for transferring liquids ranging from water based materials to heavy viscous mastics and lubricants. All of such pumps are adaptable for connection to a reciprocating driving source, such as an air motor or hydraulic motor.
Liquid supply pumps of the type described herein can be classified from one perspective as single-acting pumps or double-acting pumps. The term "single-acting pump" describes a pump wherein liquid is drawn into the pump during one direction of the driving reciprocation and is expelled from the pump during the other direction of the driving reciprocation. The term "double-acting pump" refers to a pump wherein liquid is drawn into the pump during one direction of driving source reciprocation, but liquid is expelled from the pump during both directions of driving source reciprocation. The present invention is adaptable for both types of pump, although it is described herein with reference to the double-acting class of pumps.
Certain types of liquids present particular problems for the liquid supply pumps of the type disclosed herein, especially liquids which have thermosetting characteristics which tend to cure or solidify upon prolonged exposure to air. The liquids which typify these characteristics are, among others, isocyanates which are utilized in mixed form with various types of resins, and which when applied to various substrates tend to cure out over a relatively short time period and to form an extremely hard and durable surface after curing. The pumping of this type of material, in either mixed or unmixed form, creates particular problems in the pumping equipment, for care must be taken to avoid prolonged exposure of the materials to air, for otherwise the materials may tend to cure within the pumping mechanism itself and eventually cause the pump to become inoperative.
Various types of seals and sealing arrangements have been tried in such pumps, to isolate the interior components of the pump which tend to accumulate liquid over time from the external air and environment of the pump. Since such pumps are operatively reciprocated by an elongated shaft connected at one of its ends to a pumping piston, and projecting externally of the pump at its other end, attempts have been made to design liquid and air seals about the pump drive shaft proximate the point of emergence from the pump. However, since no seal can be developed with complete liquid and air sealing characteristics over extended periods of use, it is inevitable that the pumped liquids will eventually create leakage through the seal and become exposed to air to trigger the curing process. When this occurs, the cured liquid further degrades the liquid seal, thereby causing further leakage and eventually rendering the pump inoperative.